1. Field of the Invention
This invention relates to electrophotographic developing apparatuses used with a two-component developer composed of a carrier and a toner and, more particularly, to an electrophotographic developing apparatus, in which the toner is charged by making use of tribo-frictional charging between the carrier and the toner or with a preliminary developing bias to be carried on a developing roller and selectively attached latent image areas (in case of positive development) or non-latent image areas (in case of opposite development) of a photo-sensitive drum or like image carrier.
2. Description of the Prior Art
Electrophotographic developing apparatuses, in which toner is selectively attached to latent image areas (in case of positive development) or non-latent image areas (in case of opposite development) of a photo-sensitive drum or like image carrier, or more specifically dry electrophotographic developing apparatuses, are well known in the art.
Developing apparatuses of this type are largely classified into those based on a uni-component developing process using toner alone as the developer and those based on a two-component developing process using carrier together with toner. They are also classified in dependence on whether the toner used is magnetic, that is, in dependence on whether they are based on a magnetic uni-component (or two-component) developing process or a non-magnetic uni-component (or two-component) developing process.
As an example of the magnetic uni-component developing process, a commonly termed jumping developing process has been proposed (U.S. Pat. No. 4,281,329, U.S. Pat. No. 4,292,387, U.S. Pat. No. 4,395,476 and others). In this process, a thin layer of toner is formed on a developing sleeve and brought to the proximity of the surface of a photo-sensitive drum (or like image carrier), and an AC bias is applied between the two to cause the toner to be attached to an electrostatic image.
In this jumping developing process, however, magnetic forces of a magnet roller which is accommodated in the developing sleeve cause the magnetic brush (to be formed) on the developing sleeve, the magnetic brush to be formed on the developing sleeve, and therefore a high density toner layer cannot be formed. Particularly, in its applications to full-color electrophotography or the like in which toner is attached to the entire transfer sheet surface, sufficient and homogeneous image density cannot be ensured.
The non-magnetic uni-component developing process can be advantageously applied to full-color electrophotographic apparatuses, because it is possible to use toners which are more transparent than magnetic toners. However, because of the use of the non-magnetic toner, no magnetic forces of toner can be used to supply the toner to the developing sleeve. Accordingly, a technique which additionally employs a toner feed roller has been proposed (Ricoh Technical Report, Nos. 16 and 18, 1987). FIG. 7 illustrates this technique. As shown, in this technique a toner feed roller 113 is disposed on an upstream part of a developing sleeve 112 accommodating a magnet roller 111. The magnetic forces of the magnet roller 111 are not effective to attract the toner. However, they act on a toner layer thinning blade 114, which is elastic and magnetic, thus indirectly permitting toner attraction to the developing sleeve 112. Reference numeral 115 designates a discharging brush, 117 a toner agitator, and 116 a toner hopper.
In such non-magnetic uni-component developing process, however, although indirect toner attraction to the developing sleeve 112 on the magnetic toner layer thinning blade 114 is permitted by the magnetic forces of the magnet roller 111, sufficient charge cannot be injected by the agitation of the non-magnetic toner alone by the tribo-frictional charging utilizing the toner agitator 117 or the like. In consequence, a highly dense toner layer cannot be formed on the developing sleeve 112 and, like the above case, it is impossible to ensure sufficient and homogeneous image density in the full-color electrophotography or like applications where toner is attached to the entire transfer sheet surface.
As the two-component developing process, a two-component magnetic brush developing process is usually used. In this process the toner is used together with a carrier, which is constituted by magnetic particles of iron, ferrite, etc. with or without a polymer coating layer or by magnetic fine particles dispersed in a polymer binder. A developer is formed by mixing at a fixed rate the carrier, the diameter of which is set to 50 to 200 .mu.m, and the toner, the diameter of which is set to smaller than the carrier diameter, for instance 5 to 20 .mu.m, and a fellow developer is agitated by agitating means to charge the carrier and the toner by tribo-frictional charging. The toner is attached to the carrier surface by electrostatic forces thus generated. Then, the carrier is carried together with the toner on a non-magnetic developing sleeve accommodating a magnet roller, and a magnetic brush is formed on the developing sleeve at a developing position thereof by making use of the magnetic poles (main poles) of the magnet roller.
At the developing position, the toner is selectively attached to latent image areas (in case of positive development) or non-latent image areas (in case of opposite development) on a photo-sensitive drum, by applying a developing bias to the developing position, while causing friction on the photo-sensitive drum with the magnetic brush.
In the magnetic brush developing process of this type, however, the friction of the magnetic brush, which is formed on the developing sleeve and mainly constituted by the carrier, with the photo-sensitive drum surface, is liable to disturb a toner image that is formed in the developing step or cause transfer of the carrier to the photo-sensitive drum to generate white streaks or the like.
As shown above, the magnetic developing process using a magnetic toner is disadvantageous in its applications to full-color electrophotographic apparatuses, because it is impossible to use a toner which is satisfactorily transparent.
Particularly, in the above jumping developing process the toner layer carried on the developing sleeve is low in density, and sufficient and homogeneous image density cannot be ensured in such applications where toner is attached to the entire transfer sheet surface as the full black development, photographic development or full-color development and the like.
In the two-component magnetic brush developing process, although it is possible to use either magnetic or non-magnetic toner, disturbance of a toner image on the side of the photo-sensitive drum may be caused by the friction of the carrier with the toner surface.